High precision thin-layer gas-solution microcalorimetry has been used to study the oxygen binding properties of fallow-deer hemoglobin under physiological conditions. This method measures directly the enthalpy of macromolecular ligand binding by changing the ligand activity in a manner analogous to that of the Gill thin-layer optical apparatus. By logarithmically lowering the partial pressure of oxygen we have generated differential heat binding curves of oxygen binding to fallow-deer hemoglobin in phosphate buffer at pH 7.4. The temperature dependence of the oxygen affinity was examined by generating binding curves at a number of different temperatures allowing for separation of enthalpy and free energy parameters. This type of experimental analysis makes no assumption of optical linearity between the various heme groups and reveals initially that overall oxygen binding to fallow-deer hemoglobin is less exothermic and of lower affinity than for human hemoglobin Ao. In addition, previous optical work on the ancestrally related reindeer hemoglobin (Rangifer tarandus) has indicated that the enthalpy associated with its final two oxygen binding steps is minimal. Our calorimetric determination with fallow-deer hemoglobin also reveals this tendency. Presumably, this adaptation would make it easier for these animals to maintain a consistent hemoglobin oxygen saturation level under environmental conditions where the temperature fluctuates.